This invention pertains to the radio communication art and, more particularly, to a system which controls the simultaneous transmission of information by a plurality of transmitter sites.
Simultaneous broadcast, or simulcast systems, are well known in the radio transmission art. In such systems, a plurality of remotely located transmitters simultaneously broadcast the identical audio, or information signals at a particular carrier frequency. This provides maximum signal coverage, especially in mountainous regions where large radio coverage shadows exist. A problem with such systems occurs when a receiving site is positioned between two transmitting sites such that it receives equal strength carrier signals from each. In this situation it is important that the audio message from the two transmitters be phase coherent, otherwise message reception may be lost.
The prior art has compensated for this known problem by establishing the following system. The audio signal to be simulcast is transmitted from a control center to the various remotely located transmitters. This is usually accomplished with multiplexed microwave equipment. The message includes an individual audio channel for each remotely located transmitter, with the audio on that channel predeterminedly delayed by lumped, passive delay units, known as "bricks," located in the control center. A standard brick includes lumped component circuitry which provides either 50 or 100 microsecond units of delay to audio signals. Thus, dependent upon the particular link to be set up over to the transmitter sites, a brick provided time delay may be hard wired into each transmission link, whereby the overlapping coverage zones between transmitter sites are in audio phase coherency.
As simulcast systems develop, an increasing number of remote stations are provided, whereby an increasing number of possible links between points become possible. Quite often, due to rain or some other form of interference, or an equipment failure, a link may be broken due to interruption between transmitters and, to maintain information transmission, a new link must be established. With prior art systems utilizing bricks, the establishment of new links requires substantial hand wiring of the system which, of course, is quite time consuming. Moreover, such prior art lumped constant type systems are expensive and require substantial space to house.